1. Technical Field
The present invention relates to a technology for driving a discharge lamp.
2. Related Art
As a discharge lamp used for a light source of a projector (a projection apparatus), there is known a high intensity discharge lamp (HID lamp) such as a high-pressure mercury lamp, a metal halide lamp, or a high-pressure sodium lamp. In general, the discharge lamp in the projector emits light by generating discharge light between a pair of electrodes caused by an arc discharge in response to an alternating current supply.
As one of the technologies using the discharge lamp, there is known a technology of providing a primary reflecting mirror for reflecting the discharge light toward an object on the side of one of the electrodes, a secondary reflecting mirror for reflecting the discharge light, which is emitted toward the other of the electrodes, toward the primary reflecting mirror on the side of the other of the electrodes. Since such a secondary reflecting mirror is disposed close to the electrode compared to the primary reflecting mirror, the electrode on the secondary reflecting mirror side is in the environment inferior in heat radiation performance compared to that of the electrode on the primary reflecting mirror side. Therefore, there arises the problem that the deterioration of the electrode on the secondary reflecting side is apt to be advanced due to deformation caused by excessive melting compared to the electrode on the primary reflecting mirror side.
In the past, in order for preventing the deterioration in the electrode on the secondary reflecting mirror side, there has been proposed a technology for reducing the amount of heat generation in the electrode on the secondary reflecting mirror side by making the electrical power energy supplied thereto during the period in which the electrode on the secondary reflecting mirror side functions as the anode smaller than the electrical power energy supplied thereto during the period in which the electrode on the primary reflecting mirror side functions as the anode (JP-A-2006-4919).
However, in the related art technology, although the deterioration in the electrode on the secondary reflecting mirror side caused by the excessive melting can be prevented during the initial stage of the product life cycle, there still arises the problem that in the case in which the amount of heat radiation in each of the electrodes is relatively decreased by increase in the distance between the electrodes with elapse of operating time, it becomes unachievable to melt the electrode on the secondary reflecting mirror side to the extent that the shape of the electrode can be maintained, thus the deterioration in the electrode on the secondary reflecting mirror side is even more advanced due to the deformation caused by insufficient melting.